Steam PRV Discharge Piping Layout 3

[Doiscond]

I've got a sketch attached showing the layout and dimensions, it's a steam PRV with the discharge piping routing down to the ground level instead of exhausting upward. I'm not used to seeing a relief line routed like this so I just wanted to make sure I wasn't missing something. Are there any hazards that I'm not seeing with this layout?

It looks like the outlet discharges where people could be standing under a pipe rack so that's one thing I'm going to bring up already.

Thanks!

 

[MJCronin]

Was this deadly steam pipe routing developed by your boss, the Wharton MBA ?... or is it Stanford grad ?

While your downward routing may ....occasionally,.... be acceptable for cold liquid service, discharging into a plant drain.... It is NEVER acceptable for steam systems discharging to atmosphere !!! ....

This steam discharge configuration will maim and kill people, IMHO

MJCronin
Sr. Process Engineer

 

[GBTorpenhow]

If there may be liquids present in the PSV discharge, then routing the tailpipe as a liquid/two phase relief (discharging downwards with the end of the relief tailpipe as close to grade as feasible) is preferable IMO so nobody takes a surprise shower.

 

[MJCronin]

"Outlet pipework

There are two possible types of discharge system – open and closed. An open system discharges directly into the atmosphere whereas a closed system discharges into a manifold along with other safety valves.

It is recommended that discharge pipework should rise for steam and gas systems, whereas for liquids, it should fall. Horizontal pipework should have a downward gradient of at least 1 in 100 away from the valve ensuring that any discharge will be self-draining. It is important to drain any rising discharge pipework. Vertical rises will require separate drainage. Note: all points of system drainage are subject to the same precautions, notably that valve performance must not be affected, and any fluid must be discharged to a safe location.

It is essential to ensure that fluid cannot collect on the downstream side of a safety valve, as this will impair its performance and cause corrosion of the spring and internal parts. Many safety valves are provided with a body drain connection, if this is not used or not provided, then a small bore drain should be fitted in close proximity to the valve outlet"

Excerpt from SPIRAX SARCO website ..... the venerable SS has been in the RV business for many decades

MJCronin
Sr. Process Engineer

 

[LittleInch]

It should normally have on the horizontal legs a drain slope to prevent condensation and pooling and "no pockets".

But very odd it's not connected to a larger vent line or just pointed upwards somewhere. But also need to think if the discharge will result in a shower of near boiling water as it condenses coming out of the relief valve.

Depends on the supports, guides and restraints, but the discharge force will act on the lever arm and could overstress the piping connection on the relief valve

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MJCronin]

More responses:

"But very odd it's not connected to a larger vent line or just pointed upwards somewhere."

Disagree, sir, strongly .... While it is common in Process Plants to route vent lines to some collection point, it is both normal and customary in steam and boiler plants to route sat steam dischrges up to the atmosphere. This safe preferred dischrage routing is inconformance with national code, rules and 100+ years of experience .... Steam is diferent !


But also need to think if the discharge will result in a shower of near boiling water as it condenses coming out of the relief valve.

Yes, indeed sir .... and that is why they sell these devices to be mounted on the vent piping:

The DPL Drip Pan Elbow is used to collect and remove condensate. Typically used with steam boilers, pressure relief valves, safety valves and steam pressure vessels and lines. Drip Pan Elbow shall be made of cast iron and conform to the Power Piping Code.

https://dantevalve.com/product/drip-pan-elbows/

(Competent steam piping designers should know about this, it should be shown on PIDs AND these devices have been in use for over 100 years ...)

In my opinion, it is nothing short of criminal to assign a high energy steam system design to someone who has zero experience in this area .... IT IS A LOADED GUN WAITING TO KILL SOMEONE !!!

That's why I hate MBAs .....

Comments, diagreements ??? .......................................................Anyone ???????????

MJCronin
Sr. Process Engineer

 

[Latexman]

My steam (boiler or let down station) PSV outlets are outside, in normally unoccupied areas, and point up.

Good Luck,
Latexman

 

[georgeverghese]

In engineering practice at my previous OpCo, pressurised vent releases (such as from PSV discharges) to unpressurised open drains / sewers are simply not permitted, regardless of whether they are flammable/of some HSE concern or not hazardous.
The DPL device would add another route for these condensates to be disposed off (in addition to the drain line drilled at the bottom of the PSV discharge elbow, which is an API recommendation for services such as this).

 

[MJCronin]

Even more... Some ancient history ... to boot ...

LittleInch comments: "Depends on the supports, guides and restraints, but the discharge force will act on the lever arm and could overstress the piping connection on the relief valve"

Yes, indeed !!.... and the engineering methodologies required for this dynamic loading condition evaluation is clearly defined in Appendix 2 of ASME B31.1 "Power Piping Code"

https://www.eng-tips.com/viewthread.cfm?qid=170725

https://www.eng-tips.com/viewthread.cfm?qid=90029

https://nexnor.com/wp-content/uploads/2020/02/ASME-B31.1-2018.pdf

The stress analysis of the juncture between the steam piping main and the nozzle/branch to the RV is of SUPREME IMPORTANCE to the safe and responsible engineering of the system !!

Even the esteemed and venerable CAESAR-II piping anaysis software recogizes the importance of this detailed analysis issue and offers response spectra power ...

https://docs.hexagonppm.com/r/en-US/CAESAR-II-Users-Guide/Version-13/341261

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++


Oh yeah .... I first became involved with local juncture analysis of PRV systems ..... in the mid-1980s ..

MJCronin
Sr. Process Engineer

 

[shvet]

@abot8765
Do not believe them.

Relief shall be discharged to a safe location, right? Which location to be considered as safe knows only you as this is site specific. One should provide a study to find locations nearby considered as safe and choose optimum one from both safe and cost points of view. The result should be approved by a HAZOP team. No magic, just logic, sound precautions and calculations.

Routing downward is a proper design if free draining is provided. Note that in most cases the best safe location is located high above ground to assure sound dissipation and hot gas dispersion. Safe location at ground surface in most cases is not the best idea as high plot area to be fenced and therefore creates some obstructions to personnel and vehicles. Note that even fence is not a perfect feature as humans are prone to behave another way it was intended.

Note that relief creates high forces so there is a risk that a relief case goes as not intended and the only way to find it out is to test the relief system in reality.

https://www.phmsa.dot.gov/sites/phmsa.dot.gov/files/docs/El%20Paso-Mojave%20GT%202012-5-2%20Redacted.pdf

So no additional precautions should be considered as an overkill.

 

[KernOily]

The sketch as proposed would not be my recommendation unless there is zero chance, ever, of personnel exposure to a relief event. That is a *very* hard requirement to fulfill. This might be the case in a remote unmanned operation, but even then you have to ensure that the occasional operator or craft will not be impacted during a relieving event.

Therefore, as others have noted, the PSV discharge needs to be routed away from locations where it is possible for operations or crafts to be present.

You can do a risk analysis based on your particular PHA/hazard risk ranking criteria to locate the tailpipe discharge. A silencer can be provided to attenuate the high noise level. This is an off-the-shelf item; there are several manufacturers of these.

The main hazards with steam PSV discharge are: noise, high thrust forces, and exposure of personnel to hot condensate and hot vapor phase steam. Another one folks often overlook is corrosion of the tailpipe. Atmospheric steam in lines open to atmosphere is corrosive as sh*t to carbon steel. If you get a rust-through hole in the tailpipe halfway to the outlet, someone may be exposed during a relief event. Best to make the tailpipe out of SS (expensive) or some other corrosion-resistant material.

The thrust force is best handled by a pipe stress program such as CAESAR II or Tri-Flex. CAESAR will calculate the direction and magnitude of the force based on the steam parameters you input. CAESAR will then provide the information you need to design the tailpipe restraints, to prevent thermal stress overload in the tailpipe, prevent movement, and prevent overloading the restraints that support the tailpipe.

Usually the thrust event can be assumed to be non-time-varying which means you can assume the thrust force is constant w.r.t. time and therefore you do not need to do a dynamic analysis. This is not always the case though; if it can be shown that the relief event may be time-varying, you will need to consider a dynamic event. CAESAR will handle that but you have to know or assume the parameters of the time variance.

 

[Doiscond]

Thanks all for the great info,

After talking about this proposed layout, this will be routed more in line with the designs highlighted in B31.1 Appendix II for open discharge systems, with the piping going up instead of down to grade. Stress analysis using CAESAR will determine the thrust force and restraint system used.

Again I really appreciate all the feedback and discussion!

 

[KernOily]

Sounds good. Make sure to provide a weep hole at the low point for liquid drainage out of the tailpipe.

 

[FacEngrPE]

If you are still concerned about giving someone a shower, look into adding a Exhaust Head to the discharge.

This will separate the liquid phase from the steam. Caution it is still hot.